The present invention is related to a method and apparatus for damping the oscillations of the driveline in a hybrid vehicle.
Passenger comfort and fuel efficiency have set forth increasing demands on automotive vehicle designs. It is a primary goal of most vehicle designs to provide a more efficient vehicle without having to sacrifice passenger comfort and satisfaction.
Moreover, and as alternative vehicle propulsion systems are implemented, passenger comfort and fuel efficiency are sometimes in opposition to each other. This is particularly true in hybrid vehicle designs.
A Hybrid Vehicle is a vehicle that has at least two sources of energy. A hybrid electric vehicle (HEV) is a vehicle wherein one of the sources of energy is electric and the other source of energy may be derived from a heat engine that burns diesel, gasoline or any other source of chemical energy.
Generally, a hybrid vehicle utilizes more than one type of energy storage. The HEV incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and drive vehicle systems. Numerous ways of coupling the systems may be employed and typically take the form of one or more heat engines and one or more electric drives which are coupled to a transmission to drive one or more of the vehicle""s wheels. This includes a significant complexity since the performance, responsiveness and smoothness of these devices is typically quite different. So, while an efficiency gain may be obtained from the system, the complexity of controlling it to provide the smooth response expected by the driver becomes far more difficult.
Given the high priority on efficiency in a hybrid vehicle, the task of coupling the torque sources to the wheels is well-suited to an automated manual transmission, which can be electronically controlled and has one of the highest efficiencies of any transmission device.
The transmission is positioned in the drive train between the heat engine and the driven wheels. The transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
However, the drivability of a hybrid vehicle is adversely affected due to the torque oscillations that occur when abrupt torque changes are encountered in the operation of the internal combustion engine and the transmission coupled to it. Such oscillations are encountered during shifting and launching.
For example, an automobile requires higher torque demands at low speeds for acceleration, with a decreasing demand as the cruise speed is approached.
Accordingly, and in order to meet the torque demand of the automobiles acceleration, a transmission having multiple gear ratios must be coupled to an internal combustion engine.
The combination of high driveline efficiencies, which have little damping, and more than one source of torque to be applied to the wheels, creates both problems and opportunities. This is especially apparent during shifts and the initial launching of the vehicle. Unlike an automatic, planetary style transmission, a manual transmission is unable to apply torque to the wheels during a shift, and so by itself it possesses a significant reduction in performance and driver pleasability. A solution in a parallel hybrid vehicle is to utilize a secondary torque source that is not coupled through the transmission, but is either after the transmission on the same axle, or on a different pair of wheels altogether. Therefore, by careful component design and selection, a hybrid system can be optimized for efficiency, as well as performance and smoothness.
In contrast to the torque oscillations of an automated manual transmission, an electric motor or drive train produces higher torques at startup, which decrease as increasing speed is reached.
Accordingly, and since the torque output of an electric motor is similar to demands of the vehicle, there is no requirement for a transmission or drive train used with a high-efficiency internal combustion engine. Therefore, and in order to accommodate the differences between the driving units of a hybrid vehicle, synchronization between the driving force of the two motors or drive trains is necessary.
Additionally, the damping of the torque oscillations of an automated manual transmission will further enhance the drivability and performance of the same.
Accordingly, and in order to provide a highly efficient hybrid vehicle that utilizes a fuel efficient internal combustion engine, the torque oscillations caused by a direct coupled drive train must be minimized.
In addition, hybrid vehicles also utilize a concept known as regenerative braking. Generally, regenerative braking is the conversion of the vehicle""s kinetic energy into a source of electrical power. The vehicle""s kinetic energy is converted from the spinning wheels, in response to a user request to slow or stop the vehicle. A generator is manipulated, and accordingly, produces electrical energy as it applies a stopping force to the vehicle""s axle and/or drive train in response to a stopping request.
Therefore, and in accordance with regenerative braking, the kinetic energy is converted to electric energy as the vehicle begins to slow down.
However, and in situations where the negative torque of the regenerative braking system of a hybrid vehicle is not sufficient enough to meet the driver""s braking demand request, the regenerative braking system is supplemented by a conventional friction braking system.
The use of a regenerative braking system will cause a sudden torque reversal in the traction drive system. Accordingly, and since there is a significant amount of backlash in the series of gears and couplings connecting the electric motors to the drive wheels, this torque reversal will cause an annoying disturbance known as xe2x80x9cdriveline clunkxe2x80x9d.
Therefore, there is a need for a control approach in a hybrid vehicle wherein the torque of the driveline is controlled so that the dynamics of the aforementioned backlash is accommodated in a smooth fashion, thereby preventing the vehicle""s performance from being adversely affected.
With the importance of driveline efficiencies in today""s vehicles and especially with hybrid vehicles, a manual transmission is the best solution for optimizing this efficiency. Unfortunately, a manual transmission with a standard arrangement has many shortcomings. These are mainly that it is slow to shift, has no torque to the wheels during a shift, is often rough when re-engaging the clutch after a shift, wastes part of the kinetic energy stored in engine inertia during a shift, and when automated, requires complex clutch control to attempt to perform shifts smoothly.
When the system is incorporated into a hybrid vehicle, with a secondary drive system connected to it, numerous improvements can be made. Automating the manual transmission and utilizing active speed synchronization and total torque control, allows many of the negative features to be removed. Shifts can be performed quickly without opening the clutch, smooth engagement and disengagement of each gear becomes possible, energy is recovered during upshifts, and synchronization hardware inside transmission can be removed to save cost and weight. Additionally, a hybrid vehicle can be configured such that a secondary means of applying torque to the wheels can be used, so that during a shift of the automated manual on the primary drive train, the torque is made up with the secondary system such that no change in acceleration is noted by the driver. The end result is a drive train that is highly efficient and as smooth as the best automatic transmissions.
These concepts also apply to a standard automated manual transmission connected to a single torque source such as an ICE or single electric drive, though the speed of the system is reduced and the ability to transfer torque to the wheels during a shift is lost. However, a single torque source could be used to perform the synchronization and clutch closed shifting by itself.